Field of the Invention
The invention relates to polymeric particles having a coating which provides improved anti-blocking properties, and to a method of manufacturing the coated polymeric particles. The invention also encompasses equipment used in manufacturing the coated polymeric particles.
A variety of polymeric materials are produced from an initial reaction sequence at elevated temperatures and pressures. The resulting polymeric materials exist in a molten form. The polymeric materials may be further processed directly from the molten phase, or be cooled to ambient temperature prior to further processing. Depending on processing conditions and composition of the reactants, the cooled polymeric materials may be converted to pellets or other physical forms which may be easily packaged or may be first cooled and then ground, chopped or otherwise processed prior to further processing of the material. Some polymeric materials, by the nature of their composition, tend to exhibit cold-flow properties. Examples of such polymeric materials are ethylene vinyl acetate, very low density polyethylene (i.e., polyethylene with a density of 0.90 grams per cubic centimeter or less), ethylene methyl acrylate, and ethylene n-butyl acrylate. These materials, even though previously processed to produce particulate units, such as pellets, chips or powders, will nonetheless tend to flow at ambient temperatures and pressures. The particles, after cold-flow, tend to agglomerate to form a single mass which is difficult to reinstitute into the component pellets, granules or the like. It is highly preferred that polymeric materials which exhibit these cold-flow tendencies continue to have free-flowing characteristics.
Various attempts have been made to provide a surface coating to polymeric particles to limit or eliminate the tendency to agglomerate. To this end, materials such as bisoleamide have been incorporated into the polymer reaction mixture prior to formation of the polymer particle. The bisoleamide is not miscible with the polymeric material and, in time, migrates to the surface of the particle to provide a coating which resists agglomeration. It has also been known to coat silica and talc powders onto the surface of a polymer particle to inhibit agglomeration.
It is also known to apply a micro-fine polyolefin powder coating onto polymeric pellets to decrease the tackiness of the pellets. The micro-fine powder can be applied by incorporating the powder into the chilled water of an underwater pelletizing device which cools the pellets cut after extrusion. Alternatively, the micro-fine powder can be coated onto formed polymer pellets by tumbling, airveying or the like. Also, the coating may be applied by electrostatically charging a bath of fluidized powder with an electrical potential different from that of the polymer pellets.
In the preparation of polymeric particles having improved anti-agglomeration properties, there remains a need for a product which not only exhibits superior anti-agglomeration properties following treatment with a coating material, but also retains this property after pack(aging, transport, and in further processing.